Annular thermoacoustic energy converter

ABSTRACT

A thermoacoustic energy converter for converting heat energy to electricity includes an annular resonator with a regenerator disposed therein. An electro-mechanical transducer is coupled to the annular resonator and in fluid communication with the working fluid. When heat is applied to one end of the annular resonator, a traveling acoustic wave is generated in the annular resonator causing vibrations in the working fluid that actuate the electro-mechanical transducer to generate electricity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/777,095, which claims priority to U.S. patent applicationSer. Nos. 61/176,825 filed May 8, 2009 and 61/180,367 filed on May 21,2009, the entirety of each of which is incorporated by this reference.

GOVERNMENT RIGHTS

This invention was made with government support under N00014-31-1-0543awarded by the Office of Naval Research, and W9113M-05-C-0195 awarded bythe U.S. Army Space Missile Defense Command/Army Forces StrategicCommand. The government has certain rights in the invention.

TECHNICAL FIELD

The present invention relates generally to systems and methods for thegeneration of electricity in the conversion of heat to sound in aresonator. More particularly, the present invention relates to systemsand methods for directly generating electricity in the conversion ofheat to sound in a miniaturized device which is compact light, andcapable of handling high power densities.

BACKGROUND

The concept of converting heat to sound has been known for over twohundred years. For example, in the “singing pipe,” heat is applied to aclosed end of a resonant tube having a metal mesh within the tube whichhas a “hot” end near the heated end of the resonant tube and a “cold”end further from the heat source. The terms “hot” and “cold” refer totheir relative temperatures with respect to each other. The “hot” endcould be at room temperature with the important parameter not being theactual temperature, but the temperature gradient.

An acoustical standing wave set up in the resonator tube forces aworking fluid (e.g., a gas) within the resonator to undergo a cycle ofcompression, heating, expansion, and cooling. In this case, thermalenergy is converted into acoustical energy and it maintains the standingwaves.

The work of converting heat to sound has been moved forward through thedevelopment of thermoacoustical refrigerators, as disclosed in U.S. Pat.No. 6,574,968, entitled HIGH FREQUENCY THERMOACOUSTIC REFRIGERATOR,which is incorporated herein by reference. Essentially, the conversionof heat to electricity by the present invention can be thought of as theopposite process performed by the thermoacoustic refrigerator. Thus,instead of applying energy to a piezoelectric element to thereby cool adevice, energy is being taken and converted from a heat source itself.

Early attempts to create a thermoacoustic energy converter have failedfor various reasons. For example, the process was performed in prior artdevices operating at around 100 Hz which would convert the low frequencysound to electricity. However, the process was abandoned by thoseskilled in the art because of the very low efficiency of the energyconversion process at low frequencies.

One prior art process for direct conversion of heat to electricityutilizes a permanent magnet and a moving coil. This process is costlybecause of the magnet. It is also bulky and heavy and the efficiencydecreases as the frequency of the device increases, making highfrequency operation impractical. The device itself can also causemagnetic interference with nearby magnetically sensitive devices,precluding use in certain environments.

In order to make a thermoacoustic energy conversion process practical,it may be desirable to operate the device at high frequencies. Highfrequencies can result in more efficient operation of anelectro-mechanical transducer, such as a piezoelectric element that isto be used in the present invention for the conversion of sound energyto electricity.

Another advantage of operation at high frequencies comes from acomparison with prior art thermoacoustic devices that are relativelylarge compared to semiconductor devices and biological samples. Thus, itwould be another advantage to make the thermoacoustic energy convertersmall enough to be operable with such devices and samples.

Attempts to address the shortcomings of the prior art have resulted indevices, such as that disclosed in the published International PatentApplication entitled High Frequency Thermoacoustic Energy Converter,International Publication Number WO 03/049491, which is incorporated byreference herein in its entirety. Such devices addressed the problemswith other prior art devices by using a resonator that also functions asa housing for an electro-mechanical transducer, a regenerator formedfrom random fibers comprised of a material having poor thermalconductivity and a pair of heat exchangers comprised of a materialhaving good thermal conductivity positioned on opposite sides of theregenerator. The energy converter utilizes a standing wave within eachresonator, which limits the efficiency of the device.

Another attempt is disclosed in the published U.S. patent applicationentitled Compact Thermoacoustic Array Energy Converter, U.S.Publication. Number 20090184604A1, which is incorporate by referenceherein in its entirety. In this application, a plurality of heat driventhermoacoustic prime movers are arranged in parallel, coupled by meansof an acoustic cavity to a piezoelectric electrical generator whoseoutput is rectified and fed to an energy storage element. The primemovers convert heat to sound in a resonator. The sound form aphase-locked array is converted to electricity by means of thepiezoelectric element. The generated electric energy is converted to DCby means of a rectifier set and it is then stored in a battery orsuper-capacitor. Again, this type of device generates a standing wavewithin the resonator, which limits efficiency.

In 1979, P. Ceperley introduced the concept of a traveling wavethermoacoustic device for improved efficiency. This is in contrast tothe standing wave device where a phase shift is needed to provide poweroutput. It is achieved by heat flow between a regenerator of highsurface area elements and the generated sound field, which introducesirreversibilities. In a traveling wave device there is no need for aphase shift between acoustic pressure and acoustic speed, and hence ahigh efficiency is expected. Ceperley found that acoustical energy couldbe generated by differentially heating a regenerator within a waveguide. Because of such heating, the gas in the regenerator regionundergoes a Stirling thermodynamic cycle similar to a Stirling cyclewhere air acts like a piston. Ceperley also found that if the acousticwave propagates from cold to hot inside the regenerator acousticamplification occurs; i.e. the heat gradient is driving the wave and itamplifies the acoustic power. Such a device is a thermoacoustic primemover. The regenerator replaces the stack in the standing wave device;its function is to exchange heat with the sound wave, isothermally, i.e.with ωT<<π where ω is the angular frequency of the sound wave and T isthe thermal relaxation time between regenerator and sound field.Ceperley also calculated the ideal gain and energy conversion for thesedevices. This work stimulated much research, experimentally andtheoretically, as well as numerical investigations.

Prior developments have shown the performance of a looped tube travelingwave engine. It had an average length of 2.58 m and the frequency of thegas oscillations was 268 Hz3. Further development of this type of engineby the Los Alamos group led to a device which included a resonator.Their device was 5.00 m long and it operated with helium gas at 30 atm.Its resonant frequency was 80 Hz. The experimental results demonstratedup to 50% more efficiency than its standing wave counterpart, reachingefficiencies of 41% of Carnot. A limiting factor in efficiency was theproblem of streaming which was subsequently suppressed. Due to the lowacoustic impedance of the working gas viscous losses were high. This wassolved by increasing the gas impedance by adding a standing wavecomponent.

Electronic devices and machinery produce waste heat which limits theirperformance and efficiency. Thermal management of such heat and itsconversion to electrical power would raise their output and at the sametime provide an important source of renewable energy. Achieving suchgoals with simple, efficient and high power density devices would assistin providing a solution to current energy problems. The effectiveness ofsuch an approach will be determined by the nature of the devices, ontheir ability to cope with a wide range of heat inputs from waste heat,and on their impact on the environment.

Problems that need to be solved deal with device interfacing to thesource of waste heat and device scaling to a wide range of heat sourcesincluding compact electronics. Moreover with escalating power levels inwaste heat, it is important for the devices to be high power densityunits in order to cope with high power level demands. Thus, there is anever-increasing need for more energy to be reduced by providingrenewable energy from waste heat. As there is an abundance of such wasteheat, an efficient technology is needed for converting the waste heat toelectricity. Such a technology would be capable of interfacing withsources of waste heat, would have an extended life and would berelatively inexpensive to manufacture and implement. A system or methodcapable of addressing these issues and of handling the dual function ofenergy conversion and thermal management for a wide range ofapplications would be an improvement in the art.

SUMMARY

Accordingly, the present invention provides a significant improvement inthe development of miniaturized annular traveling wave thermoacousticprime movers, operating in the frequency range of a few kilohertz. Thedimensions typically found in prior art devices have been scaled down bya substantial factor and hence the devices perform in a new frequencyand size regime. By operating with an annular geometry, positivefeedback is used to maintain the acoustic oscillations. Furtherdevelopment of this approach leads to the conversion of sound toelectricity. The generated sound power is coupled to a piezoelectricdevice for direct conversion of heat to electricity.

In addition, Particle Image Velocimetry (PIV) studies on the enginesdeveloped here show the role played by the geometric factors in thenature of the sound field inside the waveguide. These studies can beused in the design and further development of such devices. Theperformance and efficiency are analyzed within existing models of thiskind of energy conversion units.

A thermoacoustic device includes a compact annular resonant system whichconverts heat to sound in an annular resonator and which transforms thesound directly to electricity at levels which can be used to power otherelectrical systems. Heat applied to a hot end of the resonator sets up atraveling sound wave within the resonator which is coupled to asound-to-electricity converter, such as a piezoelectric monomorph energyconverter. The electrical energy at audio or ultrasonic frequencies isrectified for storage through electrical circuitry and components. Inorder to increase the energy level and thus the production ofelectricity derived from a heat source, a plurality of thermoacousticdevices may be used. A low onset diode full-wave rectifier set may beemployed with the piezoelectric device to provide a DC output for energystorage on a battery or a supercapacitor.

Injected heat generates sound in the annular resonator. The resonator isacoustically coupled to sound-to-electricity converter (i.e., the energyconverter). The chamber allows for sustained acoustic oscillations inthe resonator with positive feedback. The sound-to-electricity converteris acoustically coupled to the resonator chamber to generate maximumelectrical power from heat-generated sound. Such devices, while usefulas energy converters, may also provide thermal management in a varietyof large and small systems that produce waste heat in operation.

In accordance with one aspect of the present invention, heat from a heatsource or waste heat is coupled by a thermal conductor to the hot sideof the thermoacoustic device.

In accordance with another aspect of the present invention, an energyconverter in the form of a piezoelectric element is used to collectacoustic energy from the annular resonator.

In accordance with yet another aspect of the present invention, aplurality of energy converters are attached to the annular resonator.

In accordance with yet another aspect of the present invention, aplurality annular of resonators is coupled to a heat source forproducing electricity.

In accordance with another aspect of the present invention, audio orultrasonic frequency energy is changed to DC electricity by means of anefficient rectifier and stored in a battery or a supercapacitor.

Methods of utilizing such devices to capture waste heat from electronicdevices or machinery are also included in the present invention, as aremethods of creating such devices.

DESCRIPTION OF THE DRAWINGS

It will be appreciated by those of ordinary skill in the art that thevarious drawings are for illustrative purposes only. The nature of thepresent invention, as well as other embodiments of the presentinvention, may be more clearly understood by reference to the followingdetailed description of the invention, to the appended claims, to theappendix attached hereto, and to the several drawings.

FIG. 1 is a side view of a first embodiment of a thermoacoustic annulardevice having square corners in accordance with the principles of thepresent invention.

FIG. 2 is a side view of a second embodiment of a thermoacoustic annulardevice having rounded corners in accordance with the principles of thepresent invention.

FIG. 3 is a side view of a third embodiment of a thermoacoustic annulardevice modified for Particle Image Velocimetry measurements according tothe principles of the present invention with an expanded view of a glasstube portion of the device according to the principles of the presentinvention.

FIG. 4 is a graph illustrating an onset waveform of a 2 kHz annulardevice according to the principles of the present invention.

FIG. 5 is a perspective side view of an experimental particle ImageVelocimetry experimental setup according to the principles of thepresent invention.

FIG. 6 is a graph illustrating a traveling wave of a thermoacousticannular device according to the present invention.

FIG. 7 is a side view of a thermoacoustic annular device coupled to aPower Output Device (POD) for acoustic power extraction and conversionto electricity according to the principles of the present invention.

FIG. 8 is a side view of an array of thermoacoustic annular devicescoupled to a rectifier for storing electricity in a battery according tothe principles of the present invention.

FIG. 9 is a side view of a fourth embodiment of a thermoacoustic annulardevice modified for Particle Image Velocimetry measurements according tothe principles of the present invention with an expanded view of a glasstube portion of the device according to the principles of the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present invention relates to systems and methods related tothermoacoustic devices. It will be appreciated by those skilled in theart that the embodiments herein described, while illustrating certainembodiments, are not intended to so limit the invention or the scope ofthe appended claims. Those skilled in the art will also understand thatvarious combinations or modifications of the embodiments presentedherein can be made without departing from the scope of the invention.All such alternate embodiments are within the scope of the presentinvention. Similarly, while the drawings depict illustrative embodimentsof the devices and components in accordance with the present inventionand illustrate the principles upon which the device is based, they areonly illustrative and any modification of the invented featurespresented here are to be considered within the scope of this invention.

In view of the demands for thermal management and for energy conversionfrom heat to electricity, an efficient and compact unit, based onthermoacoustics is provided. In such a device, where heat produces soundand in the same unit the sound is directly converted to electricity, aresonant acoustic system, regenerator, an acoustic cavity, and anelectrical generator are provided.

Referring now to FIG. 1, an annular-type, traveling wave, thermoacousticenergy converter (“annular device”), generally indicated at 10 inaccordance with the present invention is illustrated. The annular device10 may be configured for operation in the 2 kHz to 4 kHz range.Likewise, the annular device may be configured to operate in theultrasonic range (e.g., about 24 kHz and higher). In these frequencyranges, advantage is taken of the high sensitivity of piezoelectricdevices and their compactness. An electro-mechanical type of transducer,such as a piezoelectric device, for sound to electricity conversion issuperior to an electromagnetic type when operated at high frequenciesand when compactness is an issue, as in the miniaturization of devices.Since the thermoacoustic devices are resonant systems, their sizedetermine the resonant frequency, and hence by miniaturizing them, theoperating frequency is raised accordingly. The choice of device size isdetermined by the application and by how much power needs to beconverted. Units consisting of arrays offer the possibility of dealingwith large power levels which maintain compactness and offer lightweightsystems.

Performance of the system of the present invention is determined byoperating conditions, mainly the temperature difference imposed by thesource of heat driving the system. The amount of heat and the resultingtemperature difference will determine the power output, its efficiencyand onset for oscillation. The larger the heat input, the higher thesound level will be in the resonator resulting in greater electricalgeneration. This allows for miniaturization of the annular device 10 forvarious applications and to extend the scope of operation ofthermoacoustics. The annular device 10 is comprised of a generallysquare-shaped annular resonator 12 having a circular cross-section(effectively forming an annular tube) having relatively square corners14-17 and defining a resonator chamber 21 therein. Of course, otherannular shapes are contemplated herein including, but not limited to,circular, oval, rectangular, hexagonal or other geometric shapes. Theregenerator 18 is comprised of material in random form, such as randomfibers of material. A hot heat exchanger 20 and cold heat exchanger 22are positioned on opposite sides of the regenerator 18. The regenerator18 includes a hot heat exchanger 18′ and a cold heat exchanger 18″ and aregenerator material 19 disposed between and in thermal contact witheach heat exchanger 18′ and 18″. The regenerator material 19 maycomprise cotton wool, glass wool, steel wool, aerogel, other fibrousmaterials, a series of perforated plastic plates or a plurality oflongitudinally aligned nanotubes. The optimum spacing between therandomly arranged fibers in the regenerator material 19, when a fibrousregenerator material 19 is utilized, may be determined by the thermalpenetration depth for the working fluid or gas, such that the acousticfield can interact thermally with each element of the regeneratormaterial 19. The regenerator 18 is positioned within the resonator 12.The hot heat exchanger 18′ is thermally coupled to the hot end 20 of theresonator 12 and thermally isolated from the cold heat exchanger 18″ andcold end 22. The resonator 12 may thus be formed of a non-thermallyconductive material to reduce thermal conduction along the annular bodyof the resonator from the hot side 20 to the cold side 22. The cold heatexchanger 18″ is thermally anchored to a fixed temperature, e.g.,ambient temperature. Both heat exchangers 18′ and 18″ may be formed as athin screen which is effectively acoustically transparent, but thatmaintains a fixed temperature at each end of the regenerator material19. The heat exchangers 18′ and 18″ may be formed from laser patternedcopper or aluminum. The working fluid is disposed within the resonator12, which also functions as the resonator or acoustic chamber 21. Inoperation, a temperature gradient is formed across the regeneratormaterial 19. The resulting temperature gradient across the regeneratormaterial 19 creates a traveling n acoustic wave within the resonator 12.The working fluid, such as air, helium, argon, combinations thereof orother known gases is disposed within the resonator 12. The working fluidallows the traveling wave to form and to activate the electro-mechanicaltransducer. Thus, when heat is applied to the first end, a travelingacoustic wave is generated in the annular resonator causing vibrationsin the working fluid that actuate the electro-mechanical transducer togenerate electricity.

A finned heat sink 24 is coupled to the cold heat exchanger to helpdissipate heat and thus cool the cold heat exchanger 22. The finned heatsink 24 may be comprised of copper, or other thermally conductivematerial, cooling fins attached to the cold heat exchanger 22 having asurface area of approximately 64.6 cm² for a 2 kHz device andapproximately 31.2 cm² for others. Such cooling fins may be sufficientfor relatively short time applications, but more efficient heatdissipation at the cold end may be needed for longer operating times.The resonator 12 is comprised of an annular waveguide 12′ that forms ahollow ring-like structure between the hot heat exchanger 20 and thecold heat exchanger 22. The annular waveguide 12′ is filled with aworking gas, such as air at atmospheric pressure (e.g., 1 atmospherepressure). The length of the annular device 10 may have an overalllength, when combining the length of each of the sides 26-29 ofapproximately 8.57 cm to 17 cm, depending on the operating frequency.

Similarly as shown in FIG. 2, an annular-type, traveling wave,thermoacoustic prime mover device (“annular device”), generallyindicated at 100 in accordance with the present invention isillustrated. The annular device 100 is configured for operation in the 2kHz to 4 kHz range. This allows for miniaturization of the annulardevice 100 for various applications and to extend the scope of operationof thermoacoustics. The annular device 100 is comprised of a generallysquare-shaped annular resonator 112, but is provided with roundedcorners 114-117. In addition, each side 126-129 of the resonator 112 isprovided with an enlarged midsection 126′ to 129′ respectively, wherethe diameter of each side 126-129 is increased to enhance resonance ofsound within the resonator 112. The regenerator 118 is comprised ofmaterial in random form, such as random fibers of material. A hot heatexchanger 120 and cold heat exchanger 122 are positioned on oppositesides of the regenerator 118. A finned heat sink 124 is coupled to thecold heat exchanger to help dissipate heat and thus cool the cold heatexchanger 122. The resonator 112 is comprised of an annular waveguide112′ that forms a hollow ring-like structure between the hot heatexchanger 20 and the cold heat exchanger 22. Opposite the regenerator118 is an energy converter connection port 125 for connecting an energyconverter, such as a piezoelectric element, thereto. The port 125 iscomprised of an opening in a side wall of the annular resonator 12 andis positioned approximately at the midpoint of the annular resonator112, that is, approximately midway between the hot heat exchanger 118′and the cold heat exchanger 118″. The annular waveguide 112′ is filledwith a working gas, such as air at atmospheric pressure (e.g., 1atmosphere pressure). The length of the annular device 100 may have anoverall length, when combining the length of each of the sides 126-129of approximately 8.57 cm to 17 cm, depending on the operating frequency.

Referring now to FIG. 3, in order to operate the annular device 100,heat is injected to a hot end 230 of the device 200 by means of aresistance wire heater 232 wound directly on the annular device 212proximate to the hot heat exchanger 220. Of course, in practice theannular device 200 would be coupled to a heat source, such as electroniccomponent, machine or other device that produces heat when operating, insuch a manner as to transfer the heat from the heat source to the hotside 230 of the annular device 200. Once a threshold temperaturegradient is reached along the regenerator 218, the device 200 emits aloud sound. The frequency of the generated sound scales with the averagelength of the annular device.

In providing a thermoacoustic energy converter where heat is convertedto sound, which in turn produces electricity via an electro-mechanicaltransducer, such as a piezoelectric transducer, the regenerator 218maintains the heat transfer process between it and the sound field atangular frequency ω as isothermal as possible, i.e. to maintain thecondition ωT<<π. Since the thermal relaxation time T is equal to r²/2Kwhere r is an effective distance between elements of the regenerator andK is the thermal diffusivity of the working gas), the isothermalcondition will be satisfied when the heat elements of the regeneratorare close to each other. This leads to spacing between elements, whichis less than δ_(K), the thermal penetration depth. δ_(K) is equal to(2K/ω)^(1/2) where K is the thermal diffusivity. In this approach, thespacing between elements should not contribute significantly to the flowresistance. The regenerator filling factor by volume may beapproximately fifteen percent (15%) and be comprised of randomlyarranged fibers of steel wool. The device can be operated at ωT<<π orwhere ωT equals approximately 0.3.

Heat is injected on the hot side 230 at the hot heat exchanger 220 by,for example, an attached heater 232 which was activated by a powersupply 234. Sound levels are monitored by a piezoresistive probe 236located inside the waveguide 238. For the PIV measurements, one section240 of the annular waveguide 238 is comprised of a transparent lucenttube, such as a PYREX® glass tube, formed from a transparent materialsuch as glass, so that a laser beam can pass through the section 240 tobe scattered by seed particles in the sound field thus providing animage of the sound field. The particles 242 may be comprised of finesmoke which is injected into the device 200 before the measurements ofparticle velocity in the sound stream are taken. The tube 241 is locatedopposite the regenerator 218, and positioned approximately halfway alongthe waveguide 238 between the ends of the waveguide 238 adjacent theregenerator 218.

The annular device 200 according to the present invention is configuredfor operating at approximately 2 kHz to produce a voltage over time. Toproduce a voltage, the annular device 200 is configured to convert someof the sound energy to electricity using a piezoelectric element. Asfurther illustrated and described herein, a relatively small tube isattached to the waveguide and is coupled a piezoelectric device, such asa monomorph piezoelectric device. Dimensions of the tube may be adjustedfor optimal impedance matching and to moderate loading of the acousticdevice. The tube and piezoelectric device effectively provides a load onthe annular device.

The lowest onset temperature difference for oscillation for a 2 kHzsquare annular engine was ΔT=60° C. when TC_(c)=55° C. and T_(h)=115° C.The generated sound wave is monitored internally by a piezoresistiveprobe, such as a Honeywell XCA401 DN. Some of the sound emanating fromthe device may be audible. For a device having a waveguide length ofapproximately 17 cm, the resonant frequency may be approximately 1.952kHz with a generated sound intensity of 0.80 W/cm². Harmonic distortionis relatively low, e.g., less than about 1%.

Referring now to FIG. 4, a sound wave, generally indicated at 300,generated at onset as a function of time for a constant heat input, isillustrated. The sound wave 300 has the characteristic initialexponential rise in amplitude followed by saturation. After the waveformis initiated at threshold, a surge of the signal is followed by aleveling off. A possible explanation of this is a rapid acousticradiation of excess heat followed by an adjustment to an equilibriumsituation. The level of sound intensity varies with heat input.

The annular devices according to the present invention may be configuredto and optimized to operate at various frequencies. For example, annulardevices may be configured to operate at 4 kHz or more. For such adevice, the circumferential average length L may be approximately 8.6cm. Such devices operate effectively above a threshold temperaturedifference along the regenerator. The oscillations can be maintained forlong periods of time (e.g., more than 30 minutes). One limiting factormay be a slow increase with time of the temperature of the cold heatexchanger due to heat influx from the hot side. Such an influx can beremedied by using more efficient cooling fins or other types of heatsinks.

An important feature of the annular device of the present inventioninvolves the generation of a traveling wave during operation of thedevice. On resonance, one would expect that the circumference of theannular device is equal to nλ (where n is 1 in and λ is the wavelengthof the generated sound), and the sound produced by the regenerator wouldcorrespond to a traveling wave in the waveguide. The impedancediscontinuity at the regenerator and air interface and possibly thetemperature gradient at the regenerator could promote a standing wavecomponent. The resonant frequency of each device corresponds to afrequency for which the circumference is equal to the wavelength λ ofthe sound with n=1.

As study of the effect of the regenerator on device performance wasperformed so as to determine the absorption of acoustic waves. For soundgenerated by a piezoelectric driver, and having a steel wool regeneratorof approximately 0.5 mm thick located between heat exchangers at eachend thereof, an average acoustic absorption of 47.8% was measured at2.55 kHz. Further tests showed that the regenerator assembly produced noreflection of the sound wave in the resonator tube and hence no standingwaves were observed; only absorption was observed. The possibility ofcausing reflected sound intensity by the regenerator with a temperaturedifference along it was investigated in the impedance tube. Fortemperature differences ΔT up to 130° C., reflected intensity off theregenerator was not observable. These experiments on the regeneratorprovide an indication of the nature of the generated sound and therelative amount of the traveling wave component in the waveguide.

Referring now to FIG. 5, an apparatus, generally indicated at 400, forimaging a sound filed inside an acoustic waveguide 402 is illustrated.The apparatus 400 utilizes particle image velocimetry (PIV)measurements. Because the annular device 401 is small, velocity imagingbecomes a real challenge. To overcome this challenge, fine seedparticles 404 of smoke are injected into the annular device 401 justprior to switching on the heating element 406 to generate sound. Themotion of the fine particles 404 in the sound field is viewed by passinga laser sheet 408 generated by a laser 410 through a glass tube section412 in the waveguide 414, which illuminates the seed particles 404 to bevisible by the camera 416. A CMOS camera 416 is directed at the glasstube section 412 to take images of the particles 404 therein. Whenequilibrium in the acoustic field is established, images can be taken bythe camera 416. The images are taken at the same time as two short laserpulses from the laser 410 are generated. The laser pulses are separatedby a predetermined, fixed interval, and the particle scattered imagesare recorded by the camera. These images or measurements are taken onthe straight glass section 412 of the annular device 401.

When employing PIV measurement techniques according to the presentinvention, the sound level output of the device may need to be held atabout 140 dB in order to maintain suspension of the seeding particles.Using such techniques, several types of flow patterns may be observed,including Gedeon streaming, start-up noise characterized byinstabilities such as vortices, and a traveling wave throughoutaccompanied by radial standing waves localized near corners of thedevice. Such measurements are typically very difficult to perform insmall high frequency devices utilizing conventional techniques. In orderto satisfy Shannon-Nyquist theorem, a 2 kHz engine would need to besampled at a minimum of 4 kHz. The method of measuring according to thepresent invention, however, allows for the measurements to be made usinga camera with a maximum frame rate per second of 300. This new methodmakes it possible to perform measurements on a flow without the need formore expensive and delicate high speed equipment, thereby makingobservation of high speed flows more readily available.

As shown in FIG. 6, the flow image of an acoustic traveling wave alongthe glass waveguide section 412 is illustrated. The image covers anaxial distance of approximately 2.5 mm. Based on a calibration of thePIV apparatus 400, particle velocity of the traveling wave isapproximately 0.15 m/sec for a 140 dB sound generated by thethermoacoustic annular device when the ΔT is approximately equal to 94°C.

Because the traveling wave component of the generated sound inside theannular device 401 does not require a phase shift between particle speedand pressure, an increase in the heat to sound efficiency is increasedover prior art devices. The efficiency can thus be measured by injectingheat by some means, such as a wire heater attached to the hot heatexchanger, and monitoring the intensity of the generated sound using apiezoresistive pressure probe which sealed inside the waveguide. Heat isgenerated in the wire heater using a power supply, such as a 9 V powersupply. Taking into account heat lost to the outside by convection,conduction and radiation, as well as to the interior of the device, thepower delivered to the hot side of the device is approximately 3.43 W.This generates a steady-state sound intensity of 156 dB inside thedevice. The working fluid is air at one atmosphere. An estimate of theefficiency for converting 3.43 W of thermal power to 156 dB of soundintensity which corresponds to acoustic power levels of approximately0.507 W. This is approximately 59.8% of Carnot. Efficiency is defined asgenerated sound power divided by direct heat power input to the hot heatexchanger. For a ΔT of 105° C. and a hot heat exchanger temperature of425K, the Carnot efficiency of a device according to the presentinvention is approximately 24.7%. Larger acoustic power values can beachieved by using larger diameter waveguides and regenerators and higherΔT.

As illustrated in FIG. 7, in order to use some of the generated soundpower for direct conversion to electricity, an annular device, generallyindicated at 500 is illustrated. The annular device 500 includes acoupler 502 in the form of a tube 503 coupled to the waveguide wall 504.The opposite end of the tube 503 is coupled to a piezoelectric element506. Such an arrangement prevents acoustic short circuiting of theannular device 500 by using a relatively small tube 503 to interface thepiezoelectric element 506 with the waveguide 510. Impedance matchingbetween the sound field and piezo 506 is provided by using a ¼wavelength coupler 502. The diameter of the coupler 502 determines theloading effect on the device 500. An overload would lead to effectivelyswitching off of the device 500. For a device having a 1.27 cm diameterwaveguide 510, the coupler tube 503 to the piezo 506 may have a diameterof approximately 3.175 mm. A PZT piezo 506 in the monomorphconfiguration is attached to the coupler 502 for converting the sound toelectricity and for monitoring the performance of the device 500. Suchan arrangement provides energy conversion from heat to sound toelectricity in a single unit.

As further illustrated in FIG. 8, it is further contemplated that anarray 600 comprised of a plurality of annular thermoacoustic devices601, 602 and 603 may be provided to convert heat from a heat source 604to audio or ultrasonic frequency energy, and from the audio orultrasonic frequency energy to DC electricity by means of an efficientrectifier and stored in a battery or a supercapacitor. Heat sinks 614,615 and 616 are provided to help maintain the cold side of eachresonator 601′, 602′ and 603′ at ambient temperature. In order toconvert the electricity generated from each energy converter 605-607 touseful energy, a transformer 609 is electrically coupled to the energyconverters 605-607. A diode bridge or bridge rectifier may be providedto convert an alternating current input into a direct current output.The electricity may be stored in a supercapacitor or battery 608 topower a light or some other electrical appliance. The storage ofelectrical energy in a supercapacitor provides a system with long cyclelives and overall superior performance to energy storage in a battery.

Each annular device 601, 602 and 603 is thermally coupled to the heatsource 604, as with a thermally conductive coupling device 610 so thatheat is by conducted from the heat source to the hot side 612 of theannular device 602. The other annular devices 601 and 603 are similarlyattached. When sound is generated inside each resonator 601′, 602′ and603′, a respective piezoelectric element 605, 606 and 607 generateselectricity that can then be stored in the battery 608. A typical arraymay consist of about 100 devices and may be linked in parallel between acold plate thermally anchored to ambient temperature and a hot platewhere heat is injected.

As further illustrated in FIG. 9, a plurality of energy converters 702,703 and 704 (energy converter 704 positioned directly behind the energyconverter 703), in the form of piezoelectric elements, may be coupled tothe annular resonator 706 of the annular device 700. In this way, asingle annular resonator 706 can provide sound energy to be converted toelectricity by a plurality of piezoelectric elements 702-704. Thepiezoelectric elements 702-704 are coupled to the resonator 706 viatubes, such as tube 708 and thus radially dispersed around acircumference of one side of the annular resonator 706.

A miniature annular thermoacoustic energy converter according to theprinciples of the present invention provides a highly efficient devicefor converting heat to electricity. Operation of the device in themid-audio frequency range (e.g., between 2 kHz and 4 kHz) demonstratesenergy conversion behavior that scales with device size and also showsperformance which can be adapted for various kinds of energy conversionapplications. Because of the dominant large traveling wave component ofthe sound generated by the present invention, a high efficiency inheat-to-sound conversion is produced. The resonant frequency correspondsto one wavelength equal to the device's center circumference. Spectrumanalysis of the generated sound, shows no standing wave components. Thetraveling wave mode is apparent as seen from measurements of theacoustic sound pressure along the waveguide using the piezoresistiveprobe. The pressure amplitude of the sound wave is independent ofposition along the waveguide circumference, suggesting that there is atraveling wave which leaves the hot heat exchanger and goes aroundtoward the cold heat exchanger. The PIV measurements of the presentinvention illustrate this as well. The magnitude of the velocity vectorsin the PIV imaging is constant over the glass tube length.

In order to further understand the nature of the generated sound wave,tests were performed on effects of the regenerator on the travelingwave. This consisted of measuring the absorption of sound by theregenerator and its reflective properties. For a regenerator 0.5 mmlong, with 85% porosity, the absorption of sound at 2.55 kHz was 47.8%.The interface between the regenerator and air did not promote a standingwave mode of the particle velocity inside the waveguide. This issubstantiated by the fact that the spectrum of the generated sound doesnot contain, within experimental error, a frequency component at afundamental frequency for a standing wave, i.e. a wavelength of 2 L.

Because gas density varies inversely with the temperature, acousticreflection at the regenerator due to the temperature differences ΔTalong the regenerator, which generates the temperature gradient thatdrives the oscillator, depends strongly on the magnitude of thetemperature difference. In the experiments leading to development of thepresent invention, the temperature difference ΔT was relatively small,being of the order of about 130° C. This explains the apparent absenceof a reflected wave component and thus no indication of a standing wave.When driven thermoacoustically, the device frequency corresponds to themode for which the pressure maximum is at the regenerator. Thus, thereis an increase in acoustic density oscillations in the hot side of theregenerator, which leads to a reduction of compliance in that region,i.e. a larger impedance of the regenerator. The speed of the gas in thesound field is controlled mainly by the reactive impedance of theregenerator. Since the operating frequency of the device is relativelyhigh, the device performance generates interesting characteristics. Forexample, an analysis of the phase shift ø around the annular deviceshows that it has two components, one phase shift along the regeneratorand another along the waveguide. Considering the regenerator, theimpedance consists of reactive and resistive components.

The compliance in the regenerator is:C=øV/γP _(O)

Where ø is the air filling factor, V is the air volume, P_(O) is theambient pressure and γ is the gas constant. The inertance L can bewritten as:L=ρl/S

where ρ is the air density, l is the length of the regenerator, and S isthe cross-sectional area.

The resistive part R of the regenerator is given by:R=6μl/Sd ² _(k)

where μ is the gas viscosity and d² _(k) is the hydraulic radius.Evaluating the above quantities for a 2 kHz device of the presentinvention, results in the following corresponding impedances ofresistance, compliance reactance and inertance, respectively:R=2×10⁵ acoustic ohmsX _(C)=2.28×10⁸ acoustic ohmsX _(L)=3.45×10⁴ acoustic ohms

The phase shift along the regenerator is approximately −89.9°, withparticle speed leading the acoustic pressure. The phase shift along therest of the device dominated by distributed acoustic inertance, alongthe approximately one wavelength waveguide, is +90°. This leads to anoverall phase shift of approximately 0 as expected for a pure travelingwave. Moreover the total impedance Z is much higher than the acousticimpedance ρc/S; in fact Z is larger than ρc/S by approximately a factorof 80. This minimizes effects of dissipation due to high acousticvelocities which is usually a problem for traveling wave devices sincelosses go as the cube of the speed. Hence by operating at highfrequencies, when the reactance is larger than the acoustic impedance ofthe waveguide, a traveling wave device leads to high efficiencyperformance. It is important to note that the high frequency annulardevice presented here works because of the positive feedback imposed bythe annular geometry. It is an acoustic oscillator whose gain A can bewritten as:A=A _(O) /A _(O)β)

where A_(O) is the gain at the regenerator and β is the amount ofpositive feedback. For an oscillator A_(O)β≧. Referring again to FIG. 4,the initial amplitude of the acoustic oscillations grows untilnon-linearities in the regenerator decrease the gain A_(O) and A_(O)βfalls to 1 for steady oscillations. The total phase shift for anoscillator is zero as shown above for the thermoacoustic device.

The annular device of the present invention provides a simple and highefficiency thermoacoustic energy converter that can operate at highfrequencies. Higher frequency can be accommodated by decreasing theoverall size of the device in order to scale it accordingly. Acousticpower gain in the regenerator, together with positive feedback,maintains the oscillations of the oscillator in order to provide simpleenergy conversion and/or heat to sound.

The energy converter of the present invention is a device which hasessentially no moving parts (other than the gas in the resonator andsound chamber and the flexing of the piezoelectric device). The gas maybe comprised of air, but may also be helium, gas mixtures or argon andhelium, or other gases known in the art. In addition, the working gasmay be pressurized for higher power density.

By operating in the mid-audio and low ultrasonic frequency ranges, thethermoacoustic devices are relatively small and can be easilypressurized to high pressure levels. The size of the resonatordetermines the frequency of the thermoacoustic engine. It does this bysetting up a traveling wave resulting from acoustic pulses generated bythe temperature gradient across the regenerator. The resonator providespositive feedback at the regenerator which sustains the acousticoscillations. The only “moving part” in the device of this invention isthe working gas which travels around the annular resonator as beingdriven by the traveling wave.

The heat-to-electricity generating system of the present invention mayincorporate various approaches to inject heat to the hot heat exchanger.A flame or a heating element can be used as the source of heat. Heat isinjected to the system by direct heating of the hot heat exchanger or byheating the hot section of the resonator to which the hot heat exchangeris thermally anchored. Heat from other sources, such as waste heat froma mechanical or electronic device, can be injected to the hot heatexchanger by metallic thermal conduction, as depicted in FIG. 8.Similarly, an appropriate radioisotope containing element may be used toinject heat to the hot heat exchanger, again by metallic thermalconduction.

The electrical generator may be a piezoelectric element in a monomorphconfiguration or bimorph configuration. Each piezoelectric element(“piezo”) is a capacitor offering high impedance for current extraction.An electrical generator may be tuned to the resonance of the resonatorto maximize electric output. Electrical power output may be maximizedwhen the electrical generator is in resonance with the acoustic systemdevice. The electric voltage obtained may be enlarged by configuring thegenerator in a bimorph mode, where two piezos attached to a metallicmembrane are connected in series; such piezos may appear as a bimetallicstrip to provide maximum voltage output when exposed to sound power.

While this invention has been described in certain embodiments, thepresent invention can be further modified with the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractices in the art to which this invention pertains.

What is claimed is:
 1. An apparatus for measuring flow of a workingfluid within a thermoacoustic energy generator, comprising: athermoacoustic energy generator comprising: an annular resonator havinga first end and a second end and defining a resonator chamber therein,the annular resonator having at least one portion formed of atransparent tubular member; a regenerator disposed between said firstand second ends of said annular resonator and in fluid communicationwith said resonator chamber; a working fluid containing seed particlesdisposed within the resonator chamber; and an electro-mechanicaltransducer coupled to the annular resonator and in fluid communicationwith the working fluid; a laser configured for directing a laser sheetthrough said transparent tubular member; a camera focused on saidtransparent tubular member configured for taking images of the seedparticles illuminated by the laser sheet within the annular resonator atthe location of the transparent tubular member; whereby the images takenby the camera can be compared to determine the movement over time of theseed particles within the annular resonator to provide measurement of atraveling wave within the annular resonator when operating.
 2. Theapparatus of claim 1, wherein the transparent tubular member iscomprised of a glass tube.
 3. The apparatus of claim 1, wherein the seedparticles are comprised of smoke.
 4. The apparatus of claim 1, whereinthe camera is a CMOS camera capable of taking images at approximately300 frames per second.
 5. The apparatus of claim 1, wherein the camerais configured to take the images at the same time as two laser pulsesfrom the laser are generated.
 6. The apparatus of claim 5, wherein thetwo laser pulses are separated by a predetermined interval so that themovement and speed of the seed particles can be determined.